The invention relates to ink jet recording devices of the ink-on-demand type, and more particularly to improvements in the printing stability of such recording devices.
As shown in FIG. 1, an ink jet recording device of the ink-on-demand type generally includes an ink ejection head 10, an ink supply tube 16 and an ink supply tank 17. Ink ejection head 10 includes a vibratory plate 15 on a substrate 14 with a nozzle orifice 12 coupled to a pressure chamber 13. A piezoelectric element 11 on vibratory plate 15 flexes in response to application of an electrical signal thereto for pressurizing ink in pressure chamber 13. This causes the ink to be expelled through nozzle orifice 12 as ink droplets 12' which are recorded on a recording medium.
In this type of ink-on-demand-type recording process, ink in pressure chamber 13 is subjected to repeated abrupt pressure changes, and tends to give rise to cavitation. This causes formation of air bubbles in pressure chamber 13. Once such air bubbles are formed, they remain trapped in pressure chamber 13 and prevent ink in pressure chamber 13 from being compressed efficiently when piezoelectric element 11 vibrates upon application of the next electrical signal. Insufficient ink compression prevents the stable formation of ink droplets 12' to be expelled from nozzle orifice 12, or prevents the formation of ink droplets themselves. This results in an interrupted printing operation or one wherein the results are adversely affected. illustrated in FIG. 1 and described above has been tested for printing operation using ink (A) saturated with air, and ink (B) deaired under reduced pressure. The test was conducted under conditions wherein an applied voltage of 100 V applied pulses having a pulse width of 100 .mu.sec, at a repetitive frequency of 2 KHz. When ink (A) was employed, after about 10 to 20 minutes following commencement of the printing operation air bubbles were produced in pressure chamber 13. Hence, stable printing could not be performed. When an ink (B) was used, air bubbles were not created after three hours of printing. This permitted stable printing operation to be carried out. The tendency to form air bubbles manifested itself to a great extent as the applied voltage and the repetitive frequency were increased.
It is known that 0.0183 ml of air is dissolvable in 1 ml of water at a temperature of 20.degree. C. and 1 atmosphere pressure. More specifically, 0.0119 ml of nitrogen is dissolvable in 1 ml of water, together with 0.0064 ml of oxygen under these conditions. These dissolved gases can be removed physically by pressure reduction, boiling, or other physical process. These physical gas removing processes are disadvantageous in that nitrogen and oxygen in the atmosphere tend to be dissolved into the ink solution over time, even though the desired properties were available at the initial stages of use.
Chemical gas removal processes have also been proposed. In these processes for chemically reducing dissolved gases, a specific chemical material is added to the ink to reduce dissolved gases, thereby lowering the concentration of the dissolved gases. An example of such chemical process is Japanese Laid-Open patent publication No. 53-61412 corresponding to U.S. Pat. No. 4,279,653. This patent teaches an ink jet ink composition, including a water-soluble wetting agent (humectant), water, a water-soluble dye and an oxygen absorber. The oxygen absorber is a sulphite oxygen absorbing material for reducing the concentration of oxygen in the ink to permit a stable printing operation. However, since the sulphites are generally not readily dissolvable in a polyhydric alcohol, or the like, which serves as a humectant for ink components, the sulphite may tend to clog the nozzle orifice. Additionally, when an oxygen reducing chemical is included in the ink composition, its reducing process also tends to discolor or change the properties of the composition.
It has also been suggested to dispose an oxygen absorbing material outside the ink tank to prevent entry of oxygen from the exterior into the ink tank. One such proposal is contained in Japanese Laid-Open Patent Specification No. 51-148,427. Under this arrangement, it is necessary to remove oxygen dissolved in the ink tank at an initial stage and the ink tank must be double-walled which results in a device which is costly to construct.
Accordingly, it is desirable to provide an oxygen absorbent device which can be utilized in an ink jet recording device to overcome the problems outlined above.